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Monday, July 23, 2018

A Tadpole Bigger than a Can of Soda --Guest Post--

Two months ago, while netting a mostly drained pond, American Museum of Natural History’s Southwestern Research Station (SWRS) volunteer Alina Downer felt something large bump into her legs as it swam through the knee-deep mud. At first, she thought it was a fish but when she reached down she was surprised to see a very large bullfrog tadpole. The team named him Goliath.

I am a herpetologist --someone who studies reptiles and amphibians-- and a graduate student at the University of Arizona staying at the SWRS for my field season and when I heard Dr. Michele Lanan boasting of Goliath’s size. I just had to see him for myself. I posted a picture of Goliath to Twitter and within hours the post had thousands of likes and hundreds of comments. I was surprised and happy about all the (mostly) positive reactions to Goliath. I was in awe of his* size like everyone else. As far as we can tell Goliath is the largest tadpole on record.

Bullfrogs (Lithobates catesbeianus) are native to the central and eastern regions of the United States. The largest of tadpoles usually grow up to six inches, and adults can be over eight inches long. Unfortunately they are not native to the western US; they were introduced there as a game species, for food (frog legs!), and to act as pest control.

In southern Arizona, bullfrogs have altered ecosystems and have been one of the main reasons for the decline of native species such as the Chiricahua Leopard Frog (Lithobates chiricahuensis) and the northern Mexican garter snake (Thamnophis eques megalops) both of which are listed as threatened. Bullfrogs eat anything they can fit in their enormous mouths, including these native species, birds, fish, and insects. They also harbor large parasite loads and the deadly chytrid fungus.

Chiricahua leopard frogs depend on aquatic habitats as eggs and tadpoles develop and adults use them for reproduction and refugia. In 2007 SWRS implemented a plan to reintroduce the Chiricahua leopard frog (listed as a threatened species in 2002) around the research station in wetlands where they had disappeared. The plan had multiple steps and included finding funding, assessing habitat, habitat restoration, starting a captive breeding program, and finally reintroduction to native habitats.From 2011-2014 more than 12,000 tadpoles had been released into seven frog ponds on the SWRS property. Thirteen other ponds were built within 10 miles of Cave Creek corridor in the hopes that the Chiricahua leopard frogs would disperse into them and they did!

But, it did not make sense to keep reintroducing leopard frogs while there were still bullfrogs around waiting to eat them. That’s where Dr. David Hall comes in. He is the field coordinator of the FROG Conservation Project and a senior wildlife biologist at the University of Arizona and has been working with his SWRS team, local landowners, and Arizona Game and Fish on bullfrog removal and Chiricahua Leopard Frog reintroduction in the Cave Creek drainage for years. The scope of the project is large: bullfrogs must be completely removed from 66 thousand acres in the San Simon Valley in Arizona. It’s a game of strategy- deciding which ponds are essential to target next to stop bullfrogs from moving into Leopard Frog territory. To fully clear the San Simon Valley, they needed to drain four major ponds simultaneously to ensure the bullfrogs cannot migrate to other water sources like natural ponds, cattle tanks, and other stream systems. The best time to remove bullfrog adults are during the evening and night when they are most active. But the team also works during the day to remove tadpoles by seining-- vertically moving a net through a water body. It is best to keep the pond dry as long as possible to ensure that the bullfrogs do not return to the area or hide in the mud, but the length of time depends on many factors including weather and landowner preferences. Once the frogs are successfully removed from the San Simon Valley they will not return, because the distance to the next bullfrog site is too far for them to move on their own.

Back to Goliath.

After capturing Goliath, the team brought it back to SWRS where the Resident Research Scientist, Dr. Michele Lanan, set the tadpole up in a 25-gallon tank and started contacting amphibian researchers to hear their opinions about the giant. Dr. Lanan says that it is hard to study isolated events like Goliath. She believes that he might have a hormonal imbalance that will prevent him from ever metamorphosing into an adult bullfrog. Based on his years of fieldwork with bullfrogs, Dr. Hall believe that Goliath is at least three years old. We are unsure exactly how long Goliath may live without metamorphosing but if it continues to grow it may get to a point where the respiratory or circulatory system will not be able to support the size of the body.

While discussing the possible research avenues with interested researchers, Dr. Lanan has been studying his growth rate, how much he is eating (he feeds on algae), and whether his behavior differs from that of normal tadpoles. Dr. Lanan has shared the news of Goliath’s discovery with other herpetology experts, many of whom were also puzzled by the cause of Goliath’s size and are interested in determining the reason for it. She hopes that research on Goliath might help solve questions about the regulation of amphibian development and paedomorphism. In the meantime, Dr. Lanan, Dr. Hall, and all the members of the frog project hope that their efforts to remove the invasive bullfrogs are successful so that native species will be able to once again thrive in the area.

The FROG Conservation Project also works on recovering wildlife and habitats in the Cienega Creek area and surrounding areas threatened by invasive species. Dr. David Hall’s team includes Jace Lankow, Sarah Wolfsiffer, Chris Prewitt, Julia Muldoon, and many other volunteers. For the last twelve years Dr. Hall has been investigating the impacts on invasive fish and bullfrogs on native Arizona species (http://frog.cienega.org). The Frog Conservation project partners with many other organizations such as the Arizona Fish and Game Department, the US Fish and wildlife Service, and the US Forest Service, and local landowners, without whom this project would not be possible.

*There is no way to tell the sex on a tadpole from morphology, but I’ll refer to Goliath as a he since he’s been given a male name.

About the Author

Earyn McGee is a PhD student at the University of Arizona where she also received her Master’s in Natural Resources. She is a NSF- Bridge to Doctorate fellow as well as an NSF- GRFP fellow. In addition to her academic endeavors like ecology and herpetology, Earyn is also interested in #Scicomm and posts to Twitter and Instagram under the handle @Afro_herper.

This post was made possible in part thanks to financial support provided by The Mindlin Foundation to David Steen to support blogging and science communication activities.